How is convergance plane distance used in side-by-side stereo 3D?
From what I know now, the convergence plane distance is the distance between the projector and the image being projected as written in blender. In fact for the anaglyph preview you can change it by clicking on the window tab/3d view and select side-by-side.

This setting seems to be used if you have some headset display. If you type the N key and go to Stereoscopy you can choose Left/Right or 3D that shows both. Now the interocular distance is the actual distance between the cameras as shown in the print screen.



Blue and red represents both cameras and the darker rectangle is the convergence plane distance



A setting that Blender suggest is the convergence plane distance/30 for the interocular distance.

Source: http://blender.org/manual/fr/render/workflows/multiview/usage.html